The GroEL-like protein of B. pertussis, the major molecular chaperone produced by this organism, was purified. This protein was found to have the tetradecameric structure typical of the GroEL family of proteins and to contain epitopes similar to other members of this family, including a human GroEL-like protein. Studies were initiated to examine the structure/function relationships of GroEL in the hope of better understanding the role that this protein plays in protein folding. An IgG1 monoclonal antibody (mAb 54G8) which binds to both B. pertussis GroEL and E. coli GroEl was produced. MAb 54G8 was found to abolish the ability of GroES to inhibit the ATPase activity of both B. pertussis GroEl and E. coli GroEL. Electron microscopy was used to map the binding site of the monoclonal antibody on the B. pertussis GroEL molecule. In the absence of the antibody, B. pertussis exhibited the tetradecameric structure typical of GroEL. Both end views (showing seven-fold symmetry of the face of the molecule) and side views were evident. When mAb 54G8 was bound, B. pertussis GroEL molecules appeared to be cross-linked together so that they formed long chains. Only side views of the molecules were seen in these long chains. When B. pertussis GroEL complexed with Fab fragments of mAb 54G8 was examined, chains were no longer observed. Instead, side views of B. pertussis GroEL were often seen with Fab fragments extending from the ends of the molecule. These data indicate that mAb 54G8 appears to bind at or near the end of the B. pertussis GroEL molecule and are consistent with a model for the GroEL-GroES complex [Creighton, T.E> (1991) Nature: 352:17-18] in which it is proposed that GroES binds to the end of the GroEL molecule.